In one corner these bags present, for access to the bag interior, a passage which when the bag has been filled enables the contained material to be tightly closed off but not sealed.
In particular, bags are known having a projecting valve formed as a prolongation of the said passage and closable to achieve complete sealing of the bag.
The valve is made of paper coated on its inside with plastic material usable for sealing the bag.
The known method uses a suitable machine to fill the bags. One embodiment of this machine presents a central rotary portion from which feed ports radially branch, other machines comprise a series of static ports. The valves of the bags to be filled are drawn over the feed ports via the passage created by the valve. In this manner each feed port pours its contents into the bag.
On termination of the filling operation, the valve is sealed by a suitable device. This sealing can be achieved by different known devices operating by ultrasound, knurling, thermal heating, etc.
Sealing can be carried out either when the bag is still on the filling port or downstream of the filling machine, during the bag processing and conveying cycle.
Generally, if the sealing operation is carried out on the bagging machine, it considerably reduces the machine production capacity. This is a considerable drawback and strongly influences the processing time and the productivity of the production cycle.
An object of the present invention is therefore to provide a device for closing bags for containing powder or granular materials while on the bag conveying line which represents an improvement on the known art in the sense that it allows sealing without slowing down the line working rate, hence increasing productive capacity to the maximum levels attainable with available bagging techniques.